These values are guidelines only. Every environment is different and requires different settings.
If you are not sure about a setting, start with a given value, increase it incrementally and check if performance improves.
Maximum memory for Webspere Liberty: For Cognos 10: 1.5 GB is sufficient in most cases. If the server has substantial memory, you can increase it to a maximum of 3 GB. There is no benefit to setting it any higher. For Cognos 11, please allow 8 GB.
Query Service Memory: Cognos 10: It is not necessary to increase this value. The query service is not used for Cognos in Changepoint. It is used by DQM (dynamic query mode) only. Cognos 11: The query service can be used by data sets/modules/dashboards. If you run into memory issue, please double the current setting.
ReportService/BatchReportService: If the server has multiple CPUs or cores, increase the number of concurrent processes. Note however, that there are no memory settings for these services.
SortBuffer: Increasing this buffer would benefit ad hoc sorting and querying. The default value is 4 MB, which could be increased by 4 to 8 times the amount, up to 32 MB.
There is also more information on the IBM web site: https://www.ibm.com/support/knowledgecenter/SSEP7J_10.2.1/com.ibm.swg.ba.cognos.crn_arch.10.2.1.doc/c_arch_performancetuning.html#arch_PerformanceTuning